The invention relates to a piston for a four-stroke internal combustion engine, comprising a piston head with a top face on the side of the combustion space, which will henceforth be referred to as piston top, and a first groove next to the piston top holding a compression ring and a second groove further apart from the piston top holding an oil scraper ring being formed in the wall of the piston head, and a piston ring land being formed between the two grooves, and further comprising a piston skirt adjacent to the piston head, whose wall includes first guide faces for guiding the piston in a cylinder, as well as a piston pin bore carrying the piston pin.
Pistons for four-stroke engines usually have more than two piston rings, to provide for adequate compression in addition to ensuring that enough oil is scraped off the cylinder walls. Piston friction and accompanying losses will increase, however, with the number of piston rings.
In U.S. Pat. No. 5,133,563 a piston is described whose piston head is provided with only two piston rings. The lower, second groove is designed to carry an L-ring whose shorter leg points towards the crankcase. This is intended to minimize oil consumption.
It is an object of this invention to further reduce oil consumption in a piston of the above mentioned type.
According to the invention this object is achieved by configuring the piston ring land between the two grooves as second guide face for guiding the piston. In addition to the piston rings this second guide face which is thus generated between the two grooves, will provide an additional seal, which will significantly reduce oil consumption compared to known piston types with two piston rings. In addition, the well-known phenomenon of piston slap will be reduced or even eliminated, and piston noise will be kept low.
In order to further improve sealing, it is proposed that the first groove be given an essentially L-shaped cross-section to hold the compression ring designed as L-ring, the longer leg of the xe2x80x9cLxe2x80x9d of the first groove extending in the wall of the piston head in radial direction, and the shorter leg of the xe2x80x9cLxe2x80x9d in axial direction, pointing towards the piston top, the long, radial leg of the first groove preferably constituting a radial guide for the L-ring. The short, axial leg of the first groove has larger dimensions in radial and axial direction than the corresponding compression part of the L-ring to be held in it. As a consequence, a gap will form between piston head and piston ring, into which compressed gas may penetrate, thus forcing the L-ring outwards. As a result, sealing will increase with an increase in pressure. The piston ring thus will require only little preloading and friction losses will be kept low.
Excellent oil scraping properties with little friction may be obtained by providing the second groove with an essentially rectangular cross-section for holding the oil scraper ring, preferably with several first oil drain passages leading into the piston interior from the bottom of the second groove. The first oil drain passages departing from the bottom of the second groove will allow the excess oil scraped off from the cylinder wall to flow towards the interior of the piston. To improve the transport of oil from the oil scraper ring to the piston interior it is proposed in further development of the invention that adjacent to the second groove on the side facing the piston skirt a bevelled surface inclined relative to the second groove be formed in the piston wall and that a plurality of second oil drain passages depart from the bevelled surface to lead into the piston interior.
For structural reasons no oil drain passages are possible in the area of the piston pin bore. To ensure oil drainage in this area as well the piston pin bore will advantageously intersect with the second groove. In this way excess oil is allowed to flow directly into the piston pin bore, thus lubricating the piston pin bearing as an additional benefit.
In order to minimize the thermal load on the compression ring a head land is formed in the piston head between the piston top and the first groove.
For further support of oil scraping from the cylinder walls it is proposed in a preferred variant of the invention that the rim of the piston skirt facing the crankcase have a sharp edge. The sharp edge of the piston skirt rim acts as an oil scraper removing the excess oil film from the cylinder wall.
To reduce friction losses it will be of advantage if the piston pin bearing is configured as a needle bearing.
The piston is preferably made of light alloy, and more preferably of aluminium alloy. Friction in the area of the guide faces is preferably reduced by providing that at least one guide face have a coating, preferably of a molybdenum compound.